- Add OPTIONS /api/volumen/* for CORS preflight (S-22).
- Invalidate session in require_login! when user no longer exists (S-8).
- Add check_csrf! to POST /admin/preview (S-1).
- Use File.realpath for symlink-safe containment in media_file (S-2).
- Memoize Users#all with mtime-based invalidation (S-12).
- Document that empty permitted_hosts means allow-all (S-5).
- Warn and fall back to random when session_key is too short (S-20).
- Enforce SLUG_REGEX (alphanumeric + dot/dash/underscore only) in creation_error
to prevent path traversal (C-1) and stored XSS (C-3).
- Add File.expand_path containment check in default_path_for as defense-in-depth.
- Escape slug in form action attributes (list, form views).
- Validate username in change_username using the same regex as create_user (C-4).
- Escape username in settings form action attributes.
- Treat submitting the current username as a no-op success instead of 'taken'.
petrbalvin